1. Field of the Invention
This invention relates generally to an adaptive method for controlling fuel delivery in a fuel cell system and, more particularly, to an adaptive method for controlling a fuel delivery injector that includes determining an error in an estimate of the fuel delivered compared to an estimate of the fuel consumed to more accurately determine a flow set-point for fuel in a fuel cell system.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode electrodes, or catalyst layers, typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). Each MEA is usually sandwiched between two sheets of porous material, the gas diffusion layer (GDL), that protects the mechanical integrity of the membrane and also helps in uniform reactant humidity diffusion. MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a by-product of the chemical reaction taking place in the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include anode side and cathode side flow distributors, or flow fields, for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Current fuel cell system software controls the injection of the anode hydrogen fuel by maintaining the pressure of the fuel cell stack using a controller with feed-forward control. The output of the controller operates to maintain the anode-side pressure of the fuel cell stack by utilizing a fuel flow set-point in moles per second. The estimated injector flow, which is based on the maintained pressure of the fuel cell stack, and which is described in more detail below, is used to estimate the concentration of nitrogen in the fuel cell stack, nitrogen bleed flow, stack anode differential pressure, anode stream water balance, and anode gas composition, as well as to detect leaks in the anode sub-system. A poorly tuned injector flow causes a large error in the nitrogen model used in the system, e.g., a 10% error in flow can cause a 20% error in the nitrogen model used. Therefore, accurately estimating injector flow is important in a fuel cell system.
As discussed above, the output of the pressure controller is used to determine the fuel flow set-point in moles per second. To set the injector's duty cycle, the maximum flow of the injector is calculated using a sonic orifice model with the injector characteristics determining a maximum flow coefficient (kv), which is assumed to be at 100% duty cycle. The fuel flow set-point is then divided by the calculated maximum injector flow to determine the duty cycle.
Using the maximum fuel flow coefficient kv inherently assumes that the injector is completely linear and has no opening and closing flow effects. However, real injectors do not behave in this manner. Furthermore, as with any production part, injectors of the same design are not identical, and production parts do not wear the same. Thus, over the course of an injector's life, it can drift from its nominal specifications. All of these potential differences from a nominal injector can cause significant differences in the calculated injector flow. Further, injector flow is important because the current control software uses it to determine nitrogen bleed flow, stack anode differential pressure, anode stream water balance, and anode gas composition. Therefore, there is a need in the art for an adaptive injector model that is capable of capturing changes and differences in individual injectors.